Greenpeace took aim at Apple, Microsoft and Amazon over how the companies are powering the data centers that house their fast-growing cloud computing infrastructures. In a move that is reminiscent of its tussle with Facebook over its first-ever data center build in Prineville, Ore., Greenpeace has zeroed in on clean energy's boogeyman, coal.

Not all big cloud computing companies are guilty of the same. In the organization's "How Clean is Your Cloud?" report (PDF), Facebook, Google and Yahoo! are hailed for their efforts to power their clouds from renewal energy sources.

In the report, Greenpeace again singles out Facebook, this time to spotlight its new green data center in Sweden.

"Facebook, one of the largest online destinations with over 800 million users around the world, has now committed to power its platform with renewable energy. Facebook took the first major step in that direction with the construction of its latest data center in Sweden, which can be fully powered by renewable energy," says the report.

The group also gave Akamai kudos for being the first IT company to report its emissions impact using the Carbon Utilization Effectiveness (CUE).

Apple Bites Back

In a statement, Apple called Greenpeace's math into question. According to company, "Our data center in North Carolina will draw about 20 megawatts at full capacity," far below Greenpeace's estimates of roughly 100 megawatts.

Moreover, the facility is "on track to supply more than 60% of that power on-site from renewable sources, including a solar farm and fuel cell installation which will each be the largest of their kind in the country."

And blunting the coal-laced criticism that first ensnared Facebook in Prineville, Ore., Apple adds that its Maiden, NC data center "will be joined next year by our new facility in Oregon running on 100% renewable energy."

In a blog post, the conservation group writes, "For a number of the facilities in the 'How Clean is Your Cloud?' report, we made estimates of power demand using fairly conservative industry benchmarks for data center investments: 1MW of power demand from servers for every $15 million, though the number is often closer to $8 million for many companies."

And in a dig at the brand's cachet, Greenpeace adds, "While Apple is well known for making more expensive consumer products, if Apple's plans for the [$1 billion] investment only generates 20MW in power demand, that would be taking the 'Apple premium' to a whole new level."

Ultimately, only Apple knows how much energy its facility in Maiden, N.C. consumes and the tech innovations the company employs to keep a lid on its electrical use.

Google, for instance, has deployed Bloom Energy's fuel cells at its main campus. The search giant is also known for using servers with non-standard features like on-board battery backups that reduce the need for big uninterruptible power supplies.

While it might stymie Greenpeace's efforts at getting a clear picture of iCloud's environmental impact, Apple looks unlikely to break the habit of keeping its technical secrets close to its chest.